I had to run a ground direct to the battery (actually the ground point next to the battery) to get the reading close and then adjust the intercept. A chassis ground wasnt good enough. The ground got me to -1.3 and then i adjusted the slope.
Vacuum reads in psi on the gauge, you have to multiply by 2.2 (i think) to get inches. Simple enough to basically double the number on the gauge to get your vacuum reading. Boost is straight psi.
2011 GT, VMP Roush TVS, 10.80 @ 127mph
DynoSpeedRacing, Memphis TN